1. Field of the Invention
The present invention is directed toward endless fabrics, and more particularly, fabrics used as industrial process fabrics in the production of, among other things, wet laid products such as paper, paper board, and sanitary tissue and towel products; in the production of wet laid and dry laid pulp; in processes related to papermaking such as those using sludge filters, and chemiwashers; in the production of tissue and towel products made by through-air drying processes; and in the production of nonwovens produced by hydroentangling (wet process), melt blowing, spunbonding, and air laid needle punching. Such industrial process fabrics include, but are not limited to nonwoven felts; embossing, conveying, and support fabrics used in processes for producing nonwovens; filtration fabrics and filtration cloths. The term xe2x80x9cindustrial process fabricsxe2x80x9d also includes but is not limited to all other paper machine fabrics (forming, pressing and dryer fabrics) for transporting the pulp slurry through all stages of the papermaking process. Specifically, the present invention is related to fabrics of the variety that may be used to mold cellulosic fibrous web into a three-dimensional structure and in making nonwoven textiles.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
Typically, the newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which may include at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each of the drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that forming, pressing and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
In the production of some paper products, such as paper towels, facial tissues and paper napkins, through-air drying for example replaces the press dewatering described above. In through-air drying, the newly formed cellulosic fibrous web is transferred from the forming fabric directly to an air-pervious through-air-drying (TAD) fabric.
Air is directed through the cellulosic fibrous web and through the TAD fabric to continue the dewatering process. The air is driven by vacuum transfer slots, hot-air blowers, vacuum boxes or shoes, predryer rolls and other components. The air molds the web to the topography of the TAD fabric, giving the web a three-dimensional structure.
After the cellulosic fibrous web is molded on the TAD fabric, it is transported to the final drying stage, where it may also be imprinted. At the final drying stage, the TAD fabric transfers the web to a heated drum, such as a Yankee drying drum, for final drying. During the transfer, portions of the web may be densified in a specific pattern by imprinting to yield a structure having both densified and undensified regions. Paper products having such multi-region structures have been widely accepted by consumers. An early TAD fabric, which created a multi-region structure in the web by imprinting the knuckle pattern of its woven structure thereon, is shown in U.S. Pat. No. 3,301,746.
A subsequent improvement in TAD fabrics was the inclusion of a resinous framework on the woven structure of the fabric. TAD fabrics of this type may impart continuous or discontinuous patterns in any desired form, rather than knuckle patterns, onto the web during imprinting. TAD fabrics of this type are shown in U.S. Pat. Nos. 4,514,345; 4,528,239; 4,529,480; and 4,637,859.
In addition, or as an alternative, to an imprinting step, the value of paper products manufactured using through-air drying may be enhanced by an embossing step, which adds visual appeal and contributes bulk, softness and extensibility to the web. The embossing step is often done as a final or near-final step, when the paper web is dry, in an embossing calender where the paper product passes through a nip formed by two rolls: one smooth and one with a patterned surface. The paper sheet will take on a degree of the pattern from the roll surface as it is pressed between the two rolls. Some sheet thickness is lost however, which is undesirable.
In other applications, the fabric may be used in the formation and patterning of wetlaid, drylaid, meltblown and spunbonded nonwoven textiles.
The present invention is an industrial process fabric designed for use as a forming, pressing, drying, TAD, pulp forming, or an engineered fabric used in the production of nonwoven textiles, which is in the form of an endless loop and functions in the manner of a conveyor. The fabric is itself embossed with the topographic features ultimately desired for the product to be manufactured. A method for embossing the fabric with the desired pattern is also disclosed.
The method for embossing the fabric envisions the use of a device having embossments thereon which are heated (or the fabric pre-heated) having two opposed elements between which the fabric may be compressed at preselected levels of compression for preselected time intervals. For example, the device may be a two-roll calender, one or both rolls of which may be engraved or etched, which allows for continuous embossing. A platen press, with upper and lower platens might also be used if the application warrants it.
An embossing medium is used which has a preselected embossing pattern, and is capable of being readily changed from one embossing pattern to another, for example, by changing the engraved calender rolls.
In addition, the embossing method provides versatility in making desired embossed fabrics for multiple applications. The properties of the desired embossed fabric depend upon the control of certain process variables under which embossing takes place and selection of fabric substrate. The process variables include time, temperature, pressure, gap setting and roll composition.